1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device by laser irradiation and to a laser irradiation apparatus.
2. Description of the Related Art
Laser irradiation is generally used in manufacturing a semiconductor device. As one of the reasons, as compared with an annealing method using radiant heating or conductive heating, the treatment time can be reduced substantially. As another reason, the laser irradiation does not thermally damage a substrate which is easily deformed by heat, such as a glass substrate.
However, a beam cross section of a laser beam has energy intensity distribution (hereinafter also referred to as intensity distribution), so that an object to be irradiated could not have been irradiated with a laser beam at a uniform intensity. For example, in the case of crystallizing an object to be irradiated by laser annealing, it has been impossible to obtain a semiconductor film with a uniform crystallinity.
Correspondingly, there is a technology in which a laser beam is split into a plurality of beams, and the split beams are combined, to obtain uniform intensity distribution of a laser beam.
This method makes intensity distribution at beam cross section uniform; however, it also cause interference between the split beams (hereinafter also referred to as split beams). Therefore, although the original intensity distribution at the beam cross section is eliminated, other intensity distribution is newly caused by interference; thus, in this case, a surface to be irradiated cannot be irradiated with a laser beam at uniform intensity.
In response to the problem, an optical path difference equivalent to a temporal coherence length (coherence length) or more is provided between the beams split using a retarding plate to suppress the interference caused by splitting of the beams (Reference 1: Japanese Patent Laid-Open No. 2003-287703).